1. Field of Invention
The present invention generally relates to photonic bandgap (PBG) crystals. More specifically, the present invention relates to an anti-reflective coating used in maximizing light transmission through the pass band of a PBG crystal.
2. Discussion of Prior Art
Prior art method stack high refractive index material and low index material alternatively whose thickness are λc/(4n) where n is refractive index of each material and λc is central wavelength at which wavelength light is reflected effectively. This periodic structure is called photonic bandgap crystal (PBG hereafter).
High reflectance can be achieved around central wavelength (stop-band). But at the same time, several photonic states are formed in the pass-band of the crystal depending on the number of layers. For some devices such as wavelength filter where some light should be effectively reflected and other light should be transmitted depending on wavelength, these photonic states are not desirable since, even for pass-band, some portion of light is reflected. For example, approximately 25% light is reflected for pass-band wavelength (0.78-1.5 μm in FIG. 1).
For simple mono-layer device, such as ordinary solar cell, reflection can be suppressed by putting λc/4 thick anti-reflection coating (ARC hereafter). But this simple concept is not applicable to PBG since PBG has 2 different kinds of bands (pass- and stop-bands) and is composed of stacked multi layers.
Whatever the precise merits, features, and advantages of the above mentioned prior art techniques, none of them achieves or fulfills the purposes of the present invention.